1. Field of the Invention
This invention relates to a multi-layer resin molded article formed by sandwich molding, and to a method of making the same.
2. Related Background Art
As the construction of a lens barrel, there is known a construction which has a lens, a lens holding frame, a fixed cylinder, an intermediate cylinder, an operating ring, etc. and in which the lens holding frame is moved and controlled in a direction parallel to the optic axis by rotation of the operating ring about the optic axis through the intermediate cylinder. As the construction of the intermediate cylinder, there is known a helicoid cylinder provided with helocoid portions on the inner and outer peripheries thereof to convert the rotational force of the operating ring about the optic axis into movement of the lens holding frame parallel to the optic axis. In the case of a lens barrel, the use of a metal material such as aluminum (Al) or brass (Bs) results in physical excellence, while on the other hand it leads to numerous problems such as heavy weight, high cost resulting from machining of the metal, and limited productivity. Injection molding of resin materials instead of metal materials has also been practised. As a resin material suitable for the helicoid cylinder, there is known polycarbonate, ABS, polybutylene terephthalate (PBT), denatured polyphenylene oxide, polyacetal or the like, and further, use has been made of resin materials having glass fiber or carbon fiber mixed therewith to increase the resin strength.
The polycarbonate material having glass fiber mixed therewith can provide a helicoid cylinder guaranteed in reduced cost, high accuracy and mechanical strength, but it still leaves a problem that the glass fiber is exposed on the inner and outer peripheral surfaces of the helicoid cylinder, whereby an improvement is required in the slidability of inner and outer cylinders slidably fitted to the helicoid cylinder.
A solution of the above-noted problem can be achieved by making the cylinder member into a multilayer structure. For example, by the sandwich molding (hereinafter referred to as the SW molding) described in Japanese Patent Publication No. 28464/1975, U.S. Pat. No. 3,809,519, U.S. Pat. No. 3,599,290, U.S. Pat. No. 3,733,156 and U.S. Pat. No. 3,873,656, a resin material with no glass fiber mixed therewith for forming the surface layer of the helicoid cylinder is first poured into the cavity of a metal mold for molding the helicoid cylinder, and then a resin material with glass fiber mixed therewith for forming the core is poured into the cavity, whereby there can be provided a helicoid cylinder having a smooth surface layer of resin and a core of resin of great mechanical strength.